A liquid crystal display unit for use in a personal computer, a cellphone, PDA, a car navigation unit, a game console and the like has a surface-emitting light source apparatus incorporated therein for coping with demands for a decrease in thickness and weight, electric power saving, higher brightness and higher definition with regard to a liquid crystal display unit. The light guiding plate has a first main surface and a flat second main surface opposed to the first main surface and is generally manufactured from a transparent material.
A light guiding plate 40 whose schematic cross-sectional view is shown in FIG. 1(A) has a first main surface 41, a second main surface 43 opposed to the first main surface 41, a first side wall 44, a second side wall 45, a third side wall 46 opposed to the first side wall 44 and a fourth side wall 47 opposed to the second side wall 45. The first main surface 41 has a surface portion provided with concavo-convex portions 42.
In a backlight-type-surface-emitting light source apparatus for use with a liquid crystal display unit, the light guiding plate 40 is arranged to ensure that the second main surface 43 thereof faces a liquid crystal display unit 60 as a schematic cross-sectional view is shown in FIG. 1(B). Light is emitted from a light source 50 and enters through the first side wall 44 of the light guiding plate 40. The light is divided into light that is reflected on the first main surface 41 and emitted from the second main surface 43 and light that is transmitted through the first main surface 41. The light transmitted through the first main surface 41 is reflected on a reflection member 51 disposed in a position facing the first main surface 41, re-enters the guiding plate 40 and is emitted through the second main surface 43. Light emitted through the second main surface 43 is guided to the liquid crystal display unit 60 that is disposed so as to face the second main surface 43. Between the liquid crystal display unit 60 and the second main surface 43 of the light guiding plate 40, generally, a prism sheet 55 and a diffusing sheet 52 are disposed in the form of a stack to uniformly diffuse light.
In a front-light-type surface-emitting light source apparatus for use with a liquid crystal display unit, the light guiding plate 40 is arranged to ensure that the second main surface 43 thereof faces a liquid crystal display unit 60 as a conceptual view is shown in FIG. 1(C). Light is emitted from a light source 50 and enters through the first side wall 44 of the light guiding plate 40. The light is reflected on the first main surface 41 and emitted from the second main surface 43. The light is passed through the liquid crystal display unit 60 arranged in a position facing the second main surface 43, and the light is reflected on a reflection member 54 and passed again through the liquid crystal display unit 60. This light further passes through a retardation film 53 and an anti-reflection layer (not shown) formed on the second main surface 43 of the light guiding plate 40 and is emitted from the first main surface 41 of the light guiding plate 40 to be recognized as an image. The front-light-type surface-emitting light source apparatus is a system that is brighter than the backlight-type surface-emitting light source apparatus and that can be kept bright with outside light alone in a day time, so that it has an advantage that its power consumption can be decreased.
Meanwhile, the prism sheet 55 is expensive and when a plurality of such sheets are used, the problem is that the number of assembling steps increases. This problem is hence overcome by forming prism-shaped concavo-convex portions 42 on the first main surface 41 of the light guiding plate 40 (for example, see JP-A-H10-55712). For attaining low power consumption and high brightness, it is required to improve the luminance efficiency by increasing the density of the prism-shaped concavo-convex portion 42 as it can be. Further, it is also under study to form a blast-embossed surface having a light diffusion effect on the second main surface 43 for removing the diffusing sheet.
Conventionally, the above light guiding plate is manufactured by an injection molding method. That is, a mold assembly having a molten resin injection portion (gate portion) and a cavity is provided, and a transparent molten thermoplastic resin is injected into the cavity through the molten resin injection portion to form the light guiding plate. It has been general practice to select the transparent thermoplastic resin from materials such as PMMA and the like. However, the amount of heat generated in a unit such as a cellphone, PDA and the like tends to increase, and a polycarbonate resin having high heat resistance is being substituted therefor.
The liquid crystal display unit for use in a cellphone and the like currently has a thickness of approximately 3 mm, and the light guiding plate has a thickness of approximately 0.7 mm when it is the thinnest. For complying with strong demands for a further decrease in thickness of a liquid crystal display unit, the light guiding plate is also required to be as thin as less than 0.7 mm.
Patent Document 1: JP-A-H10-138275
Patent Document 2: JP-A-H10-052839
Patent Document 3: JP-A-H10-055712
Patent Document 4: JP-A-H11-058406
Patent Document 5: JP-A-2004-050819
Patent Document 6: JP-A-2003-014938